A vertical heat treatment apparatus is an apparatus for a heat treatment of a semiconductor wafer (hereinafter referred to as a “wafer”), such as a CVD (chemical vapor deposition) film formation, oxidation processing or diffusion processing, in batch-type operations. This apparatus comprises a vertical reaction vessel inside a heating furnace, and a cover member which opens and closes the lower-end opening of the reaction vessel is loaded with a wafer holder thereon to hold numbers of wafers in tiers on shelves, and a predetermined heat treatment is performed after the wafer holder is carried into the reaction vessel by lifting the cover member.
In order to control the temperature inside the reaction vessel, an internal thermocouple 91 provided inside the reaction vessel 9 and an external thermocouple 92 provided near a heater 90 outside the reaction vessel 9 are used, as shown in FIG. 1, wherein the heat release value (electric power supply) of the heater 90 is controlled by a temperature controller 93 according to the comparison data between the detected temperature values and preset temperature values of these thermocouples 91 and 92. As shown in FIG. 2, the profile of the preset temperature value output by the temperature controller 93 is established wherein a temperature (Ta), e.g. in the vicinity of 600° C., is set when a wafer boat 94 loaded with wafers W is carried into the reaction vessel, and then the temperature value is increased to reach a predetermined processing temperature (Tb) which is to be maintained and then decreased to a predetermined temperature.
Hereinafter provided is an explanation of the structure of the temperature controller 93. As shown in FIG. 1, the temperature controller 93 comprises: a first computing unit 95 for carrying out a control focusing on the detected temperature value (the detected external temperature value) of the external thermocouple 92; a second computing unit 96 for cascade control; and a switch portion 97 to choose either output value of these computing units 95 and 96. Although the detected external temperature value may be used 100% for example, the first computing unit 95 may linear-interpolate the detected temperature value (the detected internal temperature value) detected by the internal thermocouple 91 and said detected external temperature value at a predetermined ratio to obtain the detected temperature value, compare this detected temperature value to the preset temperature value, and obtain the output value by integrating the deviation of the comparison result. Also, the second computing unit 96 compares said detected internal temperature value to the preset temperature value to obtain a setpoint signal by amplifying the deviation, and compares this setpoint signal to said detected external temperature value to obtain the output value by integrating the deviation.
In the process of raising the temperature Ta at carry in to reach the processing temperature Tb, firstly the power supply of the heater 90 is controlled according to the output value of the first computing unit 95, and then, by switching the switch portion 97, the power supply of the heater 90 is controlled according to the output value of the second computing unit 96.
The reason of switching the output values in this way is as follows. In the case that temperature is controlled according to the output value of the first computing unit 95, the temperature inside the reaction vessel 9 slowly rises when approaching the processing temperature Tb as a targeted temperature and converges to the temperature lower than the targeted temperature Tb due to the substantial influence of the detected external temperature value (the temperature of the heater 90). In the case that cascade control is performed on the other hand, due to the substantial influence of the detected internal temperature value, though cascade control aims to control the temperature inside the reaction vessel 9 to converges to the targeted temperature Tb, the power supply to the heater 90 is large as the deviation between the preset temperature value and the detected internal temperature value is large, which fact leads to an overshoot phenomenon that the temperature exceeds the targeted temperature Tb and then surges back to converge to the targeted temperature Tb. For this reason, in the heating process, a control (the external temperature control) is performed based on the output values of the first computing unit 95 at first, and when the temperature inside the reaction vessel 9 approaches the targeted temperature Tb to some extent, the cascade control is then performed.
The operator sets timing of switching from the control with the first computing unit 95 to the cascade control and is responsible for seeking a good timing which makes the detected internal temperature value become stable faster.
However, in some cases, when the control method is abruptly switched, the temperature might be distorted because of the impact of the switchover, and the distortion might discourage the early stabilization of the temperature. Also, when the timing of the switchover is too early, the large power supply to the heater 90 might cause overshoot due to the cascade control performed while the detected internal temperature value is still low, and the temperature inside the reaction vessel 9 takes long to become stable as a result. On the contrary, when the timing of the switchover is too late, the temperature inside the reaction vessel 9 increases slowly and the temperature inside the reaction vessel 9 takes long to become stable after all.
In operating a vertical heat treatment apparatus, the targeted values change depending on the kind of the heat treatment, wafers, etc., and in some cases, the temperature control may be adjusted after doing maintenance on the apparatus. Further, the labor to spot said timing is complicated and becomes a heavy burden to the operator since, for instance, the reaction vessel 9 has to actually be heated in order to find said timing, which heating needs to be repeated until the timing is spotted. Moreover, in a case that the adjustment by the operator is poor, the temperature inside the reaction vessel takes long to become stable, which fact decreases throughput.